The present invention relates to neutron sources, and in particular to high powered accelerator based neutron sources having liquid cooled targets.
Neutron capture therapies are two-part radiation therapies relying on the selective loading of tumor cells with a pharmaceutical containing 10B (or other isotopes with high neutron capture cross-sections) and subsequent tissue irradiation with thermal neutrons. Boron is nonradioactive until a thermal neutron is captured causing a 10B(n,a)7Li fission reaction. The resulting alpha and lithium particles are high in energy (sharing 2.3 MeV), LET, and RBE, and travel less than 10 microns in tissue. These features lead to selective tumor-cell killing provided the 10B-containing pharmaceutical localizes well in the tumor. Other nuclides with high neutron capture cross-sections could also be used.
An external beam of thermal neutrons will be able to treat 10B-loaded tumors located at, or close to, the tissue surface. However, thermal neutrons are attenuated very rapidly in tissue. The need to provide a high flux of thermal neutrons at greater depths can be realized if a neutron beam with higher than thermal energy is used. The abundance of light hydrogen in tissue allows the higher energy (“epithermal”) neutrons to be moderated through elastic collisions.
As the energy of the external neutron beam is increased, the thermal neutron flux at a given depth is increased. However, if the energy of the epithermal neutron beam is too high, the skin sparing due to 1/v reduction in the capture cross-section of 1H and 14N (and any 10B located in healthy tissues near the surface) will be offset by an unacceptably large surface dose created by protons recoiling from collisions with fast neutrons. A trade-off must be realized, then, between maximizing the thermal neutron flux at depth, and minimizing the dose to healthy tissue (particularly at the surface). The question of which energy (or range of energies) results in the optimum trade-off for a particular neutron capture therapy application is important in the development of epithermal neutrons.
Sources of energetic neutrons include nuclear reactors and particle accelerators in which energetic charged particles bombard one or more of a variety of target materials. A number of reactions have been investigated as potential sources of epithermal neutrons including 7Li(p,n) at energies between 1.88 and 3.5 MeV, and 9Be(p,n) at energies between 1.8 and 4.1 MeV. These reactions are endothermic and require high power accelerators to generate sufficiently intense neutron beams. An alternate approach is to make use of the exothermic 9Be(d,n) reaction. This reaction generates large quantities of high energy neutrons unless the deuteron bombarding energy is set below 2.0 MeV. It is then possible to design spectrum shifters (moderator/reflector assemblies) such that the final epithermal neutron beam from the 9Be(d,n) reaction is suitable for clinical use in the treatment of deep-seated tumors (Boron Neutron Capture Therapy) or rheumatoid arthritis (Boron Neutron Capture Synovectomy). Details of neutron capture therapies are disclosed in co-pending U.S. application Ser. No. 08/919,870, entitled “Neutron Capture Therapies”, which is hereby incorporated by reference.
In accelerator-based systems must be housed in a moderator/reflector assembly that is used to tailor the neutron energy to the specific treatment. The moderator/reflector assembly is typically cylindrical in cross section and contains liquid D2O moderator in a lead or graphite reflector. In order to limit the diameter of the final neutron beam targets are limited in size. Depending on charged beam particle size, power densities of 2-20 MW/m2 may be encountered in targets having areas of 10-15 cm2.
Conventional targets have been water cooled. For example, see U.S. Pat. No. 5,392,319. However, water cooled targets require high flow velocities and high flow rates in order to provide sufficient target cooling. An additional problem with water cooled targets is that high heat fluences at the target run the risk of exceeding the critical heat flux (CHF). Once the CHF is exceeded, the heat transfer significantly decreases and catastrophic system failure can follow.
Therefore, there is a need for an improved cooling system for neutron source particle targets.